In recent years, the research on high efficiency solar cells called the third generation solar cells has flourished. Among others, solar cells that use colloidal quantum dots have been reported to be, for example, capable of increasing the quantum efficiency as a result of a multi-exciton generation effect, and thus attention has been paid to them. However, in solar cells that use colloidal quantum dots (also referred to as quantum dot solar cells), the conversion efficiency is about 7% at the maximum, and there is a demand for a further increase in the conversion efficiency.
In such a quantum dot solar cell, a semiconductor film formed from an assembly of quantum dots plays the role of a photoelectric conversion layer, therefore, research on semiconductor films themselves that are formed from a quantum dot assembly is also being conducted actively.
For example, semiconductor nanoparticles that use a relatively long ligand having a hydrocarbon group having 6 or more carbon atoms have been disclosed (see, for example, Patent Document 1 (Japanese Patent No. 4425470)).
Regarding the technique for improving the characteristics of a semiconductor film formed from a quantum dot assembly, it has been reported that when the ligand molecule bound to a quantum dot (for example, about 2 nm to 10 nm) is replaced with a shorter ligand molecule, electrical conductivity is enhanced (see, for example, Non-Patent Document 1 (S. Geyer, et al., “Charge transport in mixed CdSe and CdTe colloidal nanocrystal films”, Physical Review B (2010)) and Non-Patent Document 2 (J. M. Luther, et al., “Structural, Optical, and Electrical Properties of Self-Assembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol”, ACS Nano (2008))). It has been reported in Non-Patent Document 2 that when the oleic acid (molecular chain length: about 2 nm to 3 nm) around the quantum dots of PbSc is replaced with ethanedithiol (molecular chain length: 1 nm or less), quantum dots are brought into close proximity to one another, and electrical conductivity is enhanced.